Method of manufacturing plastic film



June 9, 1953 R. e. KRESS METHOD OF MANUFACTURING PLASTIC FILM Filed Feb. 21 19 50 5 Sheets-Sheet 1 INVENTOR m Q5 no J RICHARD s KRESS ATTORNEY II I:

June 9, 1953 R. G. KRESS 2,641,022

METHOD OF MANUFACTURING PLASTIC FILM Filed Feb. 21, 1950 3 Sheets-Sheet 2 INVENTOR. RICHARD .G. KRESS v ATTORNEY R. G. KRESS METHOD OF MANUFACTURI ENG PLASTIC FILM June 9, ,1953

3 Sheets-Sheet 3 Filed Feb. 21, 1950 INVENTOR RICHARD G. KRESS ATTORNEY Patented June 9, 1953 Richard G.Kress,iLos. AngelesfCalif 'assignorto Ex'truders, Inc Culver City, Calif,, a corporation of'Califorriia Applicationzllebruaryzl, 1950,.Serial No. 145; 187

'My invention relates to theeart of making .a sheet, of tubular or .fiat1form..from thermoplastic material, and relates in .particularzto; an. improved method for -manufacturing from thermoplastic materials film, foilsheet .or thin-walled tubing.

It is. an object of the invention to improve the quality of relatively thin wallsxwhich-aremade from thermoplastic materials, -.to reduce the cost of-manufacture, increase the rate ofi'production of and simplify the equipment required for a product of higher quality .than is ordinarily obtained from accepted manufacturing procedures.

Itis an objectof the invention to provide an extrusion 'methodfor producing from :thermoplastic materiaL-athin wallsuchas a film, foil, sheet ortube, having high tensilestrength and resistance to tearing, in :longitudinaland transverse-directions, to increase the :transparency of and to consistently 'maintain thickness .of the wall within tolerances of {plus or minus-0025 in. as compared totolerances of a minimum ofgplus or minus .000.5;in. obtainable in extrusion precesses previously known to me.

It is an object of the invention to provide-ea method wherein :the thermoplastic material .is shock-coo1edat aspecified 11301111) in thezmanu facturing procedure and is drawmheth longitudinally' and transversely related to the-.shock-cooling of the thermoplastic material forming the wall.

It is an objectof theinvention tocprovide a method wherein the. molten. plasticrmaterial, ex truded in the form ofathin wall, isbroughttinto engagement with a cooled .wall surface which causes a rapid reduction'in temperature of the material and effects a change in=theimolecular structure of the material from amorphoustoward crystalline state, and wherein the wall .is stretched longitudinally and transversely'soras. to produce orientation of the moleculesin thesetwo directions, after which-the materialicomprising the Wallis shock cooledand reduced-itomoom' temperature.

A further object of the invention is to. provide a method wherein the molten plastic-materiabis extruded as a tube-whichis expanded circumferentially by application .of fiuidpressure within the tube to a selected diameter, after which the. tube is shock cooled and reduced intdiameter while maintained under longitudinal .tension.

Another object of theinvention is touprovidea plastic film of exceptionally good clarity and physical properties which is. obtained. by simultaneously and/or sequentially shock cooling, expending and drawing, .the molecules of the film being thus oriented'in two directionswhile atthe 9 Claims. (Cl. 18- 475) .2 same time being changed from amorphous ito crystalline state.

Another object of the invention-is to provide a method wherein a tube dfiplasticmaterialis issuedfrom a .die at. a rate of movement, 'isexpanded diametrically .by internal fluid pressure, and,.at.a location spacedffrom the dieis moved iorwardlyat. a rate greater than therate of movement of the tube whereitleavesthe die,.the1tube being subjected to rapid coolinga'ction atlocations along'its path of movement.

.A furtherobjectof theinvention is to provide a .method such. as defined .in vthe.'foregwising, para.- graphwherein the tube ofplastic material, after it is expanded, isibroughtinto engagement with acooledannular wallwhich-cools the'tube, limits its expansion and therefore controls the diameter .of .the tube.

A further object of the inventionis to provide amethodsuchasdefined in .theforegoingwherein the tube of plastic material, afterit is expanded,..is brought intoengagement with themner surface of a 'forwar'dlyhonverging, tubular Wall, whichcools. the wall of the tube,.limits;its expansion and .then by circumferential contractionrelieves the .plastic wall of the tube from tension -in transverse orbircurriferential direction, While the tube is moved forwardly and further cooled .towardroom temperature.

It .is aifur'ther object of 'theinvention to 1pmvideamethod whereina cooling medium is applied to thehot extruded plastic Walllimmediately after .it..has.left the extruding die, prior .to the time whenanygreat. amount of stretching of the wallihastaken pl'ace and during'the time when initial strething'of the wall occurs.

.A further -obiect or. the. invention is to.provicle a .method .whereina substance .or substances are applied toeeither or" both .theflinner and outer surfacesooftheifilm-or thin wall oiplastic. Where plasticspf tacky character are used in'the practice .of -.the invention, I,provide a method and meansfor applying toIthe inner surface of the plastic. tube a substance whichwill. prevent. sticking .of. one portionro'f .the ,plastic wall to another Whenthetubeis. flattened. .Asv afurtherexample,v the invention provides. almethod and apparatus wherebyasubstance having lubricating ,qua'lities is. appliedito the external surfaceof the tube, so that the tube will.slide.more.readily .througha tubular. stack .part of .the equipment. If .the plastic .enn'aloyedinthepractice of the invention is. of a type which is'hardened by .chemicalreaction .with .an applied "substance; my methodineludes application of such substance to *thetdbe in'the'manner hereinafter pointed'out.

Herein the term plastic is used to designate all materials suited for use in my new method and apparatus, having the quality of being at one stage of the process readily deformable and flowable in response to application of force and at another stage of the process being comparably inelastic and resisting deformation to a degree making the materials suitable for the specified services which they are to render. Among these plastics are included thermoplastic materials such as cellulose acetate, cellulose acetate butyrate, methylmethacryl-ate, polyethylene, polyvinylidene chloride (plasticized and unplasticized), polyvinyl chloride (plasticized and unplasticized), polyvinyl chloride vinyl acetate (plasticized and unplasticized), polystyrene, acrylonitrile vinyl, polyisobutylene polyethylene, copolymers of isobutylene and styrene, tetrafiuoroethylene, polyamide, blends of' rubber, synthetic and natural, with vinyl or polyisobutylenestyrene or vinyl, cellulose acetate propionate, polyvinyl alcohol, and ethyl cellulose.

Further objects and advantages of the invention will be brought out in the following part of the specification wherein I have described a specific embodiment of the invention in detail for the purpose of making a complete disclosure without limiting the scope of the invention set forth in the appended claims.

Referring to the drawings which are for illus- ,Y

trative purposes only:

Fig. 1 is a schematic, partly sectioned view showing a preferred form of apparatus forming a part of the invention and by which my preferred method may be practised.

Fig. 2 is a sectional view taken as indicated by the line 2-2 of Fig. 1.

Fig. 3 is a sectional view taken as indicated by the line 3-3 of Fig. 1.

Fig. 4 is a sectional view taken as indicated by the line 4-4 of Fig. 1.

Fig. 5 is a sectional view taken as indicated by the line 55 of Fig. 1.

Fig. 6 is an enlarged fragmentary section of the die which forms part of the invention.

Fig. 1 is a fragmentary schematic view showin means for forming a fiat tube from the plastic tube.

Fig. 8 is a schematic view showing that part of my invention pertaining to application of coating substances to the surfaces of the extruded wall of plastic material.

In the device shown in Fig. 1, I employ an extrusion screw machine 10 of conventional type having an outlet H to which a 90 elbow fitting i2 is attached, this fitting 12 having a passage l3 which receives molten thermoplastic material from the screw machine Ill under pressure and at a substantially continuous rate of flow. The passage l3 has an expanded outlet 14 facing vertically upwardly. As shown in Figs. 1 and 6, an extrusion die 15 is connected to the fitting 12. Between the die 15 and the fitting 12 there is a ring I-6 having a flaring opening 11, a spider l8 and a ring 19. The spider 18 comprises a central annular wall portion 20, a ring portion 2|, and spaced radial wall portions or spokes 22 which lie between spaced spider openings 23 adapted to connect the opening 11 of the ring IS with the opening 24 of the ring 19.

The die [5 includes the shell 25 consisting of an annular wall with a flange 26 at its lower end arranged to fit within a lip 21 which projects upwardly from a marginal portion of the ring 19. Hold-down clamps 28 engage the upper face of the flange 26, and at least three radial screws 29 are threaded through the lip 21 to engage the flange 26 for the purpose of radially adjusting the shell 25.

The shell has an upwardly flaring inner surface 30 and at the upper end of the surface 30 there is a cylindric, inwardly presented surface 3|. The die 15 further includes a die body 32 which is received within the opening of the shell 25 and has an outer conical surface 33 confronting the surface 30 and diverging downwardly with relation to the surface 30 so as to produce between the shell 25 and the body 32 an annular passage 34 which increases in cross-sectional area or enlarges downwardly, a shown. At the upper end of the body 32, there is a circular plate 35 having a cylindrical circumferential face 36 confronting the inwardly presented circumferential face 3| and being spaced therefrom so as to provide a cylindric extrusion orifice 31. Change in the cross-sectional area of the orifice 31 is accomplished by replacing the circular plate 35 with another circular plate of required diameter. So as to maintain or regulate the heat of the molten plastic material received from the screw machine Ill electrical heaters 38 and 39 are provided for the die 15. The heater 38, of slender form, is disposed in an axial opening 40 in the die body 32. The heat producin or activating medium for the heater 38, namely electric current, is carried to the heater 38 by conductor means 41 which passes through an opening in the outer ring portion 2|, a radial wall portion 23 and the central wall portion of the ring 19. The heater 39, schematically shown in Fig. 6, is annular and fits snugly around the upper portion of the shell 25. The molten plastic, when it arrives in the space or passage 34, is brought into engagement with the heated walls of the shell 25 and the body 32. By the time the plastic reaches the orifice 31, the temperature around all portions thereof is substantially equalized and the rate of flow of molten plastic through all portions of the orifice 31 is constant, within very close limits, this condition contributing in part to the production of a superior plastic tube or sheet.

The die body 32 has at its lower end a cylindrical portion 32a which extends into the circular opening 24 of the ring 19 and defines therein an annular chamber of large cross-sectional area at the lower end of the upwardly and outwardly flaring annular passage 34. A conical body 2011 is secured to the lower face of the central portion 20 of the spider l8 and projects downwardly into the opening l1 and the flared outlet portion 14 of the passage [3. An air tube 42 extends radially inward through an opening 43 in the spider, and its inner end is connected to the lower end of a tube 44 which projects from the upper face of the land plate 35 of the die body 32 down through the body 32 to an elbow passage 46 in the central wall 20 of the spider 18.

The space immediately adjacent the upper face of the die I5 comprises the first shock coolin zone and has therein the shock cooling ring 41. Above the shock cooling ring 41, a shock cooling member 48 identifies the second shockcooling zone of the device, and above the member 48 a third shock cooling member 49 identian finternal .diameter corresponding to "the diameter of the wall surface it, Fig. 6, of the die L5.

The second shock cooling member is comprises :a conical wall .51 which converges up- .wardly. Gther walls 52 cooperatewith thewall -5! S01aSftO iorma chamber-53 adapted to re- 'ceive a coolant such as cold iWPLlZEl' which is delivered 'to the bottom of the chamber .53 by a circular perforated distributing pipe d tto which :water is fed through piping eiiunder control of .a thermostatic valve 3% associated with :ther

mometer 5? located in the chamber The thirdgshock coolingrnember $419 comprises a water chamber formed in part by the lower portion 'of a-metal-tube orsstaclcbli having an'internal diameter substantially the-same as the internal diameter of ,theupper end of the conical wallet. Thermetal wall of the stackbtis cylindrical and to the upper portion thereof a continuous flow of .water is fed from outlet openings in an encircling water distributing ring 55, led through piping thunder control of a thermostatic valve ill and associated thermometer iii disposed in the water chamber 63 or the shock cooling member Immediately above the stack 533 there is a forming memberor stacir iit a lower portion 6d of which is circular. The metal wall of the stack 53, above the lower end 954 is gradually flattened on: opposite sidesso that the upper of the staclrliidefines a narrow diametrically arranged mouth (it as shown in Fig. 3. Above the mouth there are cooperating large and s all pinch- ,off rollers 81 and 58. Variable reed e means ts are provided for the roller ti, this drive means ell being schematically indicated as a variable speed gear mechanism adapted to be driven through a chain iii and adapted to drive the roller iii through chain and racket means 'H.

In accordance with conventional extrusion practice the screw machine it is heated and delivers the molten thermoplastic material continuously to the die it, the result being that a tube l2, Fig. l, or" the thermoplastic.material is discharged upwardly from the die orifice. This tube 72 extends up through the parts ll, 43, iii, 58 and G3 and in flattened form passesbetween the rollers ti andfiii and travels over the top of the roller 6? to other equipment which will be hereinafter described. Under control of the pressure reducing valve l3, fluid under pressure, such as air, is delivered from a suitable source through the piping 42 and i l into the interior ofthe tube t2, inflating and expanding this tube 72 so as to stretch itswall diametrically thereby increasing the diameter of the tube 12 and decreasing the thickness of its wall. This trans- Verse stretching of the wall of the plastic tube 12 produces transverse molecular orientation. However the variable transmission 39 is adjusted so that the surface speed of the roller 57 will beg eater than the surface speed of the wall of the tube 62 where it leaves the die orifice. Therefore, the tube is. stretched, and subjected to molecular orientation, in longitudinal direction. -In this way the wall of the tube 72 is given increased strength and resistance to tear in both transverse and longitudinal directions.

In addition to the foregoing, the quality of the product is very greatly increased bythe shock cooling of the plastic walls of the tube '12 at prescribed locations along .the path oftravel of the tube during itsformation. As thetube 12 leaves the orifice of the die hi, internal 'air'pressure forceszitout against the conical W211- to of the cooling ring 41, causing :a quick :reduction in temperature of the wall of .thetube 1.2 at this .point, ;so :that there immediately :initiated a conversion of the'molecules of the plastic material from lamorphous toward crystalline state. For example, if the thermoplastic; material forming' the-tube 71.2.is polyethylene, the temperature of the material passing :through the die lfiwmay be approximately 375 F. The shock cooling ring 4? ismaintained atxaltemperatureof around 210 Ft, with theresult that the'wall'of the tube 12, immediately after being issued from the orificeof the die :15 is reduced to a temperature veryclose'to thevtemperaturezof the cooling ring 5?, namely 210 l t, it is stretched transversely and longitudinally and at the same time issubjected to a first stage of shock cooling. As the wall of the tube i2 moves upwardly from the shock coolingring 41, internal air pressure ex pands it as shownat l2a,-so thatthe wallet-the tube 12 in this expanded condition is brought into engagement with the internal surface of the tubular wall 5| wherein it is again shock cooled by direct contact with the cold Wall 5-,! having a temperature of approximately F. lh-e wall 5! is reduced in diameter in the direction of movement of the wall of the tube 32. Therefore it constrictsthe tube above the horizontal line of engagement of the tube Wallwith the wall El and relieves the tube wall from transverse or circumferential tension. The taper of the wall ti, in the present practice of the invention is 1 in. to the ft... but may, under various circumstances be within the range of 4 iii/ft.

The stretched Wall of the tube 12 passes upwardly from the upper end of the tapered wall 5| into the lower end of the stack 58 whereinit is subjected to the action of the third shock cooling means 49 which sets the plastic after it has been subjected to limited stretching while moving vertically across the space between the upper end of the wall 5| and the lower end of the wall 53. ,By set is meant the reduction of the temperature and state of the plastic wall to a point where it is substantially relieved of elasticity and therefore will stretch only an insignificant amount aiterleavingthe zoneof the third shock cooling means at. The wall of the tube is substantially completely cooled by the time it reaches the lower end .of the flattening stack member 53. By use of the method and apparatus heretofore disclosed, a thin durable and transparent plastic wall is rapidly and economically formed. It is a feature of the present process to stretch the plastic wall to substantially equal extent in both transverse and longitudinal directions and to stretch the Wall in both transverse and longitudinal directions with acoeificient of 5, that is to say the wall .of the tube 12, after it issues from the annular orifice oi the die 55, is stretched to 5 times its initial diameter and is stretched to 5 timesits original length. Therefore, the area of the wall '12 issued from the die i5 is increased 25 fold. If the wall thickness of the tube ?2 within the orifice of the die it is..-03 in., the thickness of the tube wall issued from upper end of the stack or shell 62% will have a thickness of .0012 in. withina tolerance of plus or minus .00025 in.

The members 48, All and 63 are vertically adjustable with relation to each other and with relation to the die liand the first shochcooling means 4'1. A vertical structure schematically indicated :at it, has thereon :slidable .braoliets 75 for supporting the forming-stack parts 49, 58, and the shock cooling member 48.

From the roller 61, the flattened tube is carried.

through gusset forming means 16 which is follower by means 11, referred to as pinch off rollers 18 and 19, for preventing escape of air from the interior of the tube during the gusset forming operation. As shown in Figs. 1, 4 and 5, the gusset forming means 16 has walls 80 which diverge from a point near the roller 18 toward the roller 61, and walls 8I which connect the edges of the walls 80 as shown, thereby cooperating with the walls 80 to form a hollow wedgeshaped tube, gusset folding members 82. Each of these gusset folding members 82 comprises a rib projecting inward from the center line of the wall 8I and gradually increasing in projection, from the front toward the rear end of the gusset formin means 16. Between the spaced sealing means represented by the rollers til-68 and. 18-19, the plastic tube is inflated. A hypodermic needle 83, attached to an air hose 84, is caused to puncture the wall 12b of the tube, to deliver into the interior thereof a body of air or bubble. The separated or expanded walls of the tube pass into the gusset forming means 16. At the front end 85 of the gusset forming means 16, the tube, as indicated at 120 in Fig. 1, assumes substantially the rectangular cross sectional configuration defined by the front ends of the walls 88 and 8|. The wall 120, shown in Fig. 4, has wall portions 12d lying adjacent the walls 80 and wall portions 12c lying adjacent the walls 8|. As the wall 120 travels forwardly (downwardly) from the front end 85 of the device '16, the rib members 82, of increasing projection, fold the central portions of the wall portions 12c inwardly, while convergence of the walls 80 move the wall portions 12d toward each other, thereby forming gussets 81 from the wall portions 12c, these gussets lying between wall portions 12d when the tube leaves the gusset forming device 16 as shown at 12 in Fig. l. The tube passes under the rollers 18 and 18. It then passes downwardly to larger rollers 88, at least one of which is driven, and whereby pulling tension is applied to the gusseted tube. Between the rollers 19 and the rollers 88, the gusseted tube passes over rollers 89, some of which serve as guides another of which apply to the bag a flattening or smoothing effect.

From the rollers 88 the gusseted tube is carried across guide rollers 90, 9| and 92 to transverse sealing means 93 and then to rotary cut-off means 84 which cuts off the sealed lengths of the tube and to a chute 95 delivers flat gusseted bags 96, sealed at one end and open at the other.

The sealing means 98 comprises simultaneously rotating cylinders 91 having thereon projecting bars 88 which periodically engage opposite sides of the gusseted tube, pressing the plastic walls thereof together and sealing the same transversely of the tube. Then, as the tube travels forwardly, the cut-off means 94 acts to cut off the sealed section of the tube. This cut-off means 94 comprises cylinders I and IOI driven respectively by a chain and sprocket means I02 from rotating parts associated with the cylinders 91. The cylinder I00 carries a projecting knife blade I03, and the cylinder IOI carries a back-up bar I04 of metal softer than that of the knife blade I03.

The invention provides means for producing flat sheet from the plastic tube which issues from the upper end of the member 63, Fig. l. The

gusset forming means 16 is replaced by a tube smoother or flatter I05, such as shown in Fig. 7. This spreading and flattening means I05 comprises upper and lower plates I08 and I01 which are disposed on the opposite sides of the path of movement of the plastic tube 12b from the roller 81 to the roller 18, and which converge toward each other. The flattened tube, after passin over the roller 51 as indicated at 12b, is inflated between the plates I06 and I01, the air pressure within the tube portion 12b, spreading this tube laterally so that it will be smooth and flat when it leaves the lower ends of the plates I06 and I01 and passes under and around the roller 18. The flattened tube is passed over the roller 19 and across rollers 89 to the puller rollers 88. It then passes under the roller and over the top of roller 9|. Rotary cutter discs I I 0, one of which is shown in elevated position with relation to the roller 9| in Fig. 1, are lowered so as to cut off the edges of the flattened tube where it passes over the roller 9|. This cutting operation severs the top and bottom portions of the flattened tube and they move onwardly through the apparatus as separate flat sheets indicated by dotted lines III and II2. These sheets pass over rollers I I3 and H4 to suitable means of disposal, such, for example, as the rolls IIS and H1 on which the sheets III and II2 are wound.

In order to avoid complication in disclosure of the principal elements of the invention, I have in Fig, 8 separately shown that part of the invention pertaining to the application of substances to the surfaces of the plastic tube during the formation thereof. In Fig. 8, I schematically show the extrusion die 9 with the tubular plastic wall 12 extending upwardly therefrom and expanded into contact with the conical wall 5I. Within the tube 12 there is a vertical pipe I20 having on the upper end thereof a spray head I 2|. An injection nozzle I22 extends into the lower end of the pipe I20, so that the action of an air jet from the nozzle I22 into the pipe I20 will produce a continuous circulation of air from the interior of the tube 12 and into the lower end of the pipe I20, as indicated by arrow I23.

The interior of the tube 12 forms a part of a closed circulation system including a pump I24 having its inlet connected by piping I25 and a duct I26 in the die 9 with the interior of the tube 12. The outlet of the pump is connected through a chambered member I21, piping I28 and a duct I29 in the die 9 with the nozzle I22. When the pump I24 is in operation, there is a continuous flow of air into the tube 12 through the nozzle I22 and the spray head I2l, and then out of the tube 12 through the duct I26. The ducts I26 and I29 are extended through radial webs or spokes 22 in the die structure.

The chambered device I21 comprises a means for adding a coating substance to the circulating stream of air, or other gaseous or vaporous fluid which is fed by the pump I24 into the device I21. The device I21, in one practice of the invention, comprises an insufilator for adding a powder, such as talc, for example, to the air stream, so that this powder will be sprayed out onto the inner surface of the tube 12 as indicated by arrows I30 in Fig. 8. The gaseous pressure within the tube 12 is maintained and controlled by connecting a source of gaseous pressure I3I with some part of the air circulating system, for example, the chambered device I21, through a pressure regulator I32 which operates whenever the pressure in the system drops below a predetermined value to feed additional fluid medium into the air circulating system. A pressure relief valve 134 is connected. to the system for bleeding 05f excess pressure should rise in temperature in the circulating system cause a pressure increase beyond a predetermined value. A pressure E39 is connected into the system to indicate the existing pressure.

In Fig. 8, I show the first shock cooling member 41, and around this member I have provided an annular tray I98 to hold a liquid, for example water or other lubricant i239. An annular Wick M9 is disposed immediately above the shock cooling member 4'! so as to the external surface of the tube 72. Fingers i l! support this wick Mil so that a portion thereof extends into the liquid I39 in the annular tray I39. The liquid I99 is applied to the lower portion of the wick Hit and by capillary action is carried through the wick I49 to the surface of the tube H as such tube 72 moves upwardly. The liquid thus applied to the external surface of the tube 72 acts. in the example of the invention now being described, a lubricant which enables the tube "5?. to slide easily through the opening defined by the conical wall 5|. Also, the liquid applied. to the outer surface of the tube 72 by the wick Hi9 may act as a means for preventing the tube 72 from sticking to the conical wall 51.

I claim:

1. In a method of forming a wall from thermo- 1 plastic material of the character described: heat-- ing the material; continuously forming a sheet of the heated material; moving the sheet con tinuously along a path of movement; bringing all portions of the sheet consecutively into engagement with a cooled wall surface so as to extract heat therefrom; and stretching the sheet after it has been so cooled so as to produce molecular orientation therein.

2. In a method of forming a wall. from thermoplastic materal of the character described: heating the material; continuously forming a sheet of the heated material and moving the sheet forwardly through a path of movement as it is formed; and simultaneously bringing the full. width of the sheet into engagementwith a cooled Wall surface so as to extract heat therefrom, and stretching the sheet so as to produce molecular orientation therein while it is in engagement with said wall.

3. In a method of forming a wall from thermo plastic material of the character described: heat log the material; extruding a tube of the heated material; moving the tube forwardly; shockcooling the Wall of the tube; stretching said wall to a selected diameter; and circumferentially restraining further expansion of said tube while cooling the said wall to near its substantially non-plastic state.

l. In a method of forming a wall from ther mcplastic material of the character described: heating the material; extruding a tube of the heated material; moving the tube forwardly; stretching the tube and increasing its diameter; flattening the tube at a place spaced from the position at which the tube is extruded, thereby forming a substantially closed air chamber within the tube wherein air pressure may act to expand the tube; forming a continuous closed path for the flow of a gaseous medium, said path including the interior of said tube; circulating a gaseous medium through said path; maintaining said gaseous medium under pressure and adding to said gaseous medium a substance which is to be applied to the inner surface of said tube.

5. In a method of forming a wall from thermoplastic material whic his set at normal temperature: extruding a sheet of the material heated to thermoplastic condition; rapidly cooling the sheet to a temperature between the temperature at which it is extruded and said normal temperature; stretching the sheet; and again rapidly cooling the sheet while at the same time subjecting it to a stretching force, to effect longitudinal molecular orientation therein, to bring the sheet to said normal temperature and to desired dimension.

6. In a method of forming a wall from thermoplastic material which is set at normal temperature: extruding a tubular sheet of the material heated to thermoplastic condition; rapidly cooling sheet to a temperature between the temperature at which it is extruded and said normal temperature; then stretching the sheet laterally so as to effect lateral molecular orientation therein; and again cooling the sheet While at the same time subjecting it to a longitudinal stretching force, to effect longitudinal molecular orientation therein, to bring the sheet to said normal temperature and to desired dimension.

'7. In a method of forming a wall from thermoplastic material which is set at normal temperature: extruding a tubular sheet of the material heated to thermoplastic condition; bring the sheet into contact with the surface of a cooled body so to rapidly bring the sheet to temperature between the temperature at which it is extruded and said normal temperature; stretching the sheet principally laterally; restraining the sheet from lateral stretching; stretching the sheet longitudinally; and cooling the sheet to said normal temperature.

In a method of forming a wall of thermoplastic material: moving forwardly a tube of thermoplastic material at formative temperature; expanding the tube to increase its diameter; moving the expanded portion of said tube through and in contact with a cooled annular wall; and stretching said tube longitudinally.

9. In a method of forming a wall of thermoplastic material: moving forwardly a tube of thermoplastic material at formative temperature; quickly cooling the tube; expanding the tube to increase ts diameter; moving the expanded portion of said tube through and in contact with a cooled annular wall; stretching said tube longitudinally while controlling its diameter; and cooling the tube to a temperature at which it can be handled without deformation of the tube wall.

RICHARD G. KRESS.

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